Britain's Financial Times once described Steven Pinker as "a handsome man" with a hairstyle that "works equally well for Led Zeppelin front man Robert Plant." But even if the Harvard psychologist didn't look like a rock star, he would still play to packed houses on the lecture circuit. He has something rare among top-tier scholars, an ability to convey complex ideas with clarity, flair and wit. That's one reason his books — most recently, The Blank Slate — make best-seller lists even as they make waves in academia. The other reason is those waves in academia. Pinker is on the forefront of an intellectual sea change.

Decades of social-science dogma depicted the human mind as having few built-in features — kind of like a computer with no programs, a blank slate. Pinker, along with others in the young field of evolutionary psychology, disagrees. For starters, he argued in The Language Instinct, we have a genetically based word processor, engineered by natural selection. Among the other legacies of natural selection, say the new Darwinians, are such impulses as jealousy and vengefulness. So Pinker draws fire from those who ascribe all ills to the corruption of pristine souls. But evolutionary psychology has a brighter side: love and compassion are also in our genes. Besides, Pinker notes, biology isn't destiny. "Nature," he quotes Katharine Hepburn's character in The African Queen as saying, "is what we were put in this world to rise above."

Every half-century, it seems, an eminent Harvard psychologist crystallizes an intellectual era. Near the end of the 19th century, William James, writing in Darwin's wake, stressed how naturally functional the mind is. In the mid-20th century, after a pendulum swing, B.F. Skinner depicted the mind as a blank slate. Now the pendulum is swinging again. Harvard, which lured Pinker from the Massachusetts Institute of Technology last year, seems poised to keep its tradition alive.